Craft beer sales have been growing by double digits, even as overall beer sales have flattened. And several independent craft beer makers — all based in the Western U.S. — are expanding production to the East. But to keep the flavor true, they have to tinker with beer's main ingredient: water.

Every day, a half-dozen employees of Oskar Blues Brewery file into a small room in Brevard, N.C. It's cluttered with boxes, petri dishes and test tubes.

This isn't quite a chemistry classroom. There are beer cans and pint glasses. It's a tasting area, where a sensory panel meets daily to try the beer. David Morris is scribbling notes about what he and his colleagues taste.

The staff also does blind tests, comparing its North Carolina-made beer with samples from Colorado, where the company started.

"When we first started brewing here, yeah, there was a remarkable difference, but now in a given tasting panel it's really hard to pick out the two," says Eric Baumann, director of fermentation and quality control.

That's because in the two years since bottling in the East began, the brewery has been manipulating the water. The water used in Colorado comes from mountain runoff, collected out of ponds. In North Carolina, the brewery uses local municipal water.

"It's very similar; however, typically, when you're looking at water, it's also how the city is treating the water. And what we found here is there's quite a bit more chlorine. And so we've had to put in a charcoal filtration to remove the chlorine," Baumann says.

Oskar Blues now produces almost half of its beer in North Carolina. That means easier and cheaper shipping to East Coast cities, and fresher beer in new markets. This area around Asheville, N.C., is quickly becoming a sort of Napa Valley of beer.

Also expanding east is Sierra Nevada, the nation's second-largest craft brewery. It has outgrown its original plant in California. Brian Grossman runs Sierra Nevada's new facility in North Carolina and is also working with the water. As The Salt has reported, it's critical to match the flavor profile customers want.

"You know, that's what defines a great brewer — consistency. There's plenty of people out there that make these wonderful beers, but can they make it every single time?" he says.

Before opening the plant earlier this year, Sierra Nevada took water samples from all over the region and analyzed them. It realized it needed to essentially add salt and prepare for fermentation.

Overall, the changes are subtle, and customers may never recognize them.

"I don't think it's so much about whether people can tell that the water has been treated so much as the brewers themselves knowing they're putting out a reliable and consistent product," says Andrew Fisher, who works at a nearby beer store and conducts weekly tastings.

Fisher suggests another motivation for all the attention to water: "It's a part of brewing at this point of the game because it's become such a competitive and huge enterprise."

Craft beer is only about 8 percent of the American beer market, but that's double its share compared with 15 years ago. And growth is continuing with the nation's third-largest craft brewer. New Belgium has outgrown its original facility in Fort Collins, Colo. It's building a new plant in North Carolina as well. And it'll be doing what it can to make the local water replicate Rocky Mountain runoff.

The craft beer market has been growing even as overall beer sales have been flat. And several independent craft beer makers based in the West are setting up production in the East. But there's a big challenge. The water used to brew the beer tastes different. North Carolina Public Radio's Jeff Tiberii has more.

JEFF TIBERII, BYLINE: Every day, a half dozen employees of Oskar Blues Brewery file into a small room in Brevard, North Carolina. It's cluttered with boxes, petri dishes and test tubes. This isn't quite a chemistry classroom. There are beer cans and pint glasses. It's a tasting area where a sensory panel meets daily to try the beer. David Morris is wearing cargo shorts and sports a bushy beard. He's scribbling notes about what he and his colleagues taste.

DAVID MORRIS: Buttery character or a green apple character or maybe a burnt flavor in a beer.

TIBERII: Staff also does blind tests, comparing North Carolina-made beer with samples from Colorado where the company started. Eric Bauman is director of fermentation and quality control.

ERIC BAUMAN: When we first started brewing here, yeah, there was a remarkable difference. But now in a given tasting panel, it's really hard to pick out the two.

TIBERII: That's because in the two years since bottling in the East began, they've been manipulating the water. The water used in Colorado comes from mountain runoff, collected out of pawns. In North Carolina, the brewery uses local municipal water.

BAUMAN: It's very similar. However, typically, when you're looking at water, it's also how the city is treating the water. And we found here there is quite a bit more chlorine, and so we've had to put in charcoal filtration to remove the chlorine.

TIBERII: Oskar Blues now produces almost half of its beer in North Carolina. That means easier and cheaper shipping to East Coast cities and fresher beer in new markets. Also expanding east is Sierra Nevada - that's the nation's second largest craft brewery. It's outgrown the original plant in California. Brian Grossman run Sierra Nevada's new facility in North Carolina.

BRIAN GROSSMAN: You know, that's what defines a great brewer is consistency. There's plenty of people out there that make these, you know, wonderful beers, but can they do it every single time?

TIBERII: Hence the focus water, beer's main ingredient. Grossman walks through the expansive plant outside of Asheville. Before opening the plant earlier this year, Sierra Nevada took water samples from all over the region and did analysis.

GROSSMAN: This right here is a dosing system - right here. Calcium chloride right here that we're dosing in line. This is the dosing station. So we're just going to impregnate it in there. We're going to go to our mash water and we're going to go.

TIBERII: In other words, add some salt and prepare for fermentation. Overall, the changes are subtle, and customers may never recognize them. Andrew Fisher is a beer enthusiast who works at a local store where he conducts weekly tastings.

ANDREW FISHER: I don't think it's so much about whether people can tell that the water has been treated so much as the Brewers themselves knowing that they're putting out a reliable and consistent product.

TIBERII: Fisher suggests another motivation for all the attention to water.

FISHER: It's a part of brewing, at this point in the game, because it's become such a competitive and huge enterprise.

TIBERII: Craft beer is only about eight percent of the American beer market, but that's double its share compared to 15 years ago. Growth is continuing with the nation's third largest craft brewer. New Belgium has outgrown its facility in Fort Collins, Colorado. It's building a new plant in North Carolina, as well. And it'll be doing what it can to make the local water replicate Rocky Mountain runoff. For NPR News, I'm Jeff Tiberii in North Carolina. Transcript provided by NPR, Copyright NPR.